Higher performance, lower cost, and greater density of integrated circuits are ongoing goals of the computer industry. In processing these integrated circuits on semiconductor substrates, layers are often deposited on the substrate and subsequently some or all of the layers are removed. One process for material removal from a substrate is plasma etching.
In plasma etching, plasma is utilized to assist etch processes by facilitating an anisotropic removal of material along fine lines, within vias, within contacts, and for other general patterning operations on a semiconductor substrate. Examples of such plasma assisted etching include reactive ion etching (RIE), which is essentially an ion activated chemical etching process.
In a plasma etching process reactive species are generated as a plasma from a bulk gas. The reactive species diffuse to a surface of a material being etched and are adsorbed on the surface of the material being etched. A chemical reaction occurs, which results in the formation of a volatile by-product, which is desorbed from the surface of the material being etched and diffuses into the bulk gas, where it can be purged from the reaction chamber.
Many plasma reactors provide energy to a gas in the reactor chamber by coupling Radio Frequency (RF) electric power into the chamber. The RF power ionizes, dissociates, and excites molecules within the plasma body. In particular, the RF power provides energy to free electrons in the plasma body. Ionization may occur from an energized free electron colliding with a gas molecule causing the gas molecule to ionize. Dissociation may occur from an energized free electron colliding with a gas molecule, such as O2, causing the molecule to break into smaller molecular or atomic fragments, such as atomic oxygen. Excitation occurs when the collision, rather than breaking molecular bonds, transfers energy to the molecule causing it to enter an excited state. Control of the relative amounts of ionization, dissociation, and excitation depends upon a variety of factors, including the pressure and power density of the plasma. Due to ionization, the plasma body typically includes substantially equal densities of negatively and positively charged particles.
Plasmas may be particularly useful for anisotropic etching of a semiconductor substrate. Anisotropic etching is etching that occurs primarily in one direction, whereas isotropic etching is etching that occurs in multiple directions. Anisotropic etching is desirable for manufacturing integrated circuit devices, because it can be used to produce features with precisely located sidewalls that extend perpendicularly from the edges of a masking layer. This precision is important in devices that have feature sizes and spacing comparable to the depth of the etch.
To accomplish an anisotropic plasma etch, a semiconductor substrate such as a wafer may be placed in a plasma reactor such that a plasma forms in an electric field perpendicular to the substrate surface. This electric field accelerates ions perpendicularly toward the substrate surface for etching. One conventional approach to anisotropic plasma etching uses parallel planar electrodes. Often, the lower electrode acts as a pedestal for a wafer. RF power is applied to the electrodes to produce a plasma and accelerate ions toward the substrate surface.
The crystalline silicon or thin insulating layers of some modern integrated circuit designs may be damaged by high energy ion bombardment, so it may be necessary to decrease the RF power applied to the electrodes for lower ion energy etch processes. Decreasing the RF power, however, will reduce the ion density in the plasma. Decreased ion density usually decreases the etch rate.
Inductively coupled plasma reactors have been used with an RF coupling mechanisms to generate the plasma and control the ion density and ion bombardment energy. Power is applied to an induction coil surrounding the reactor chamber to inductively couple power into the chamber to produce the plasma. The inductively coupled power accelerates electrons circumferentially within the plasma. As a result, the charged particles generally do not accelerate in any specific direction. To move ions toward a substrate some type of bias is typically applied between the substrate and the plasma. In other words, a separate source of power may be needed and applied to a substrate support to accelerate ions toward the substrate for etching. A relatively high level of power may be applied to the induction coil to provide a plasma with a high ion density, and a relatively low level of power may be applied to the substrate support to control the energy of ions bombarding the substrate surface. As a result, a relatively high rate of etching may be achieved with relatively low energy ion bombardment.
While low energy ion bombardment may reduce damage to sensitive layers of the integrated circuit, other problems may be encountered that interfere with the anisotropic nature of the etch. In particular, low energy ions may be deflected by charges that accumulate on the substrate or mask surface during etching causing a charge buildup.
This charge buildup may result from the relatively isotropic motion of electrons in the plasma as opposed to the anisotropic motion of the ions. The normal thermal energy of the plasma causes the electrons to have high velocities because of their low mass. These high velocity electrons collide with molecules and ions and may be deflected in a variety of directions, including toward the substrate surface. While the negative bias on the substrate tends to repel electrons, the high velocity of some electrons overcomes this negative bias. The electrons are deflected in a variety of directions and have a relatively isotropic motion. As a result, electrons deflected toward the substrate surface tend to accumulate on elevated surfaces of the substrate or mask layer, rather than penetrating to the depths of narrow substrate features.
Ions, on the other hand, have a large mass relative to electrons, do not have high random velocities, and are directed toward the substrate in a perpendicular direction. This anisotropic acceleration allows ions to penetrate to the depths of narrow substrate features more readily than electrons.
As a result, negatively charged electrons tend to accumulate on the upper surfaces of the substrate or mask layer, while positively charged ions tend to accumulate in the recessed regions of the substrate that are being etched. These accumulated charges may form small electric fields, often referred to as “micro fields,” near features on the surface of the substrate. While these small electric fields may have little effect on high energy ions, they may deflect low energy ions used in low energy etch processes for small integrated circuit features. The negative charge on the substrate or mask surface tends to attract positively charged ions, while the positive charge in recessed regions tends to repel these ions. As a result, low energy ions falling into recessed regions between features may be deflected into feature sidewalls, thereby undercutting the mask layer. This undercutting can degrade the anisotropic etch process and inhibit the formation of well-defined features with vertical sidewalls.
Because of these issues, and others, neutral beam etching processes have been proposed. In a neutral beam process, the ions are accelerated toward the substrate, but they then pass through a variety of proposed mechanisms to neutralize the ions by supplying electrons to the ions. The neutralized ions then strike the surface of the substrate with an amount of kinetic energy related to the mass and velocity of the neutralized ions. That kinetic energy is enough to cause a chemical reaction on the surface of the substrate. In other words, the chemical process, such as an etching process, at the substrate is activated by the kinetic energy of the incident neutral species.
These previously proposed neutralizing structures take different forms. As an example, one proposal includes a deflection plate that deflects the ions as they move toward the substrate by a small angle in one direction then another deflection plate deflects the ions back to a perpendicular direction relative to the substrate. These deflection plates are negatively charged such that the ions can readily pick up electrons and neutralize as they strike the plates and before they strike the substrate.
In another example of a previous proposal, one or more grids are disposed between a region where the plasma exists and the substrate. These grids may be configured to emit electrons such that as the ions travel through the grids and toward the substrate these emitted electrons are available for recombination with the ions to create neutral species for impact with the substrate.
However, these previously proposed neutral beam etching apparatuses require additional elements to be added to the reaction chamber to neutralize the ions. These additional elements add complexity and cost to the chamber as well as new elements that must be maintained. Moreover, as semiconductor wafers become very large, such as 300 millimeter wafers and above, these additional elements generally must span an area even larger than the wafer which may introduce structural problems with how to build the neutralizing structures without any physical deformation across the large area.
The inventor has appreciated that for the reasons stated above, and for other reasons, there is a need in the art for alternative and simplified structures and processes for generating neutral beam flux for processing substrates.